Copper complex for capturing carbon dioxide

ABSTRACT

A composition and method are disclosed for capturing CO 2 . A pollutant/greenhouse gas can be converted into a valuable organic compound using a nontoxic reducing agent. CO 2  may be economically captured from the atmosphere, from products of combustion, or from byproducts of various chemical processes. The method reduces CO 2  in a three-step reaction cycle, in which a binuclear metal-organic compound converts CO 2  to oxalate in a redox cycle under mild conditions.

This is the United States national stage of international application PCT/US2014/060249, international filing date Oct. 13, 2014, which claims the benefit of the Oct. 14, 2013 filing date of U.S. provisional patent application Ser. No. 61/890,403 is claimed under 35 U.S.C. § 119(e). The complete disclosure of the priority application is hereby incorporated by reference in its entirety.

This invention was made with Government support under Louisiana EPSCoR award number EPS-1003897 awarded by the National Science Foundation. The United States Government has certain rights in the invention.

TECHNICAL FIELD

This invention pertains to a composition and method for reducing carbon dioxide to oxalate.

BACKGROUND ART

An economical route for converting carbon dioxide to value-added organic compounds would be highly desirable because of the role carbon dioxide (CO₂) plays in global climate change, and in the depletion of fossil fuel resources. Although CO₂ is an inexpensive, non-toxic, abundant carbon feedstock, it is difficult to economically reduce CO₂ to a more useful form because of its thermodynamic stability and kinetic inertness. For CO₂ reduction to be attractive on a large scale, the process needs to work under mild reaction conditions, and the process must be economical.

CO₂ has traditionally been captured by absorption into a solution of an organic amine. This method is energy-intensive; it requires heating the solution to disperse the absorbed CO₂ for storage. If the absorbed CO₂ is simply driven off, then it must be stored somewhere (e.g. in an underground rock formation) to avoid release into the atmosphere.

Other methods that have been tried include activating and reducing CO₂ by electrochemical and electrocatalytic means in the presence of various transition metals and alloys.

There have been reports of using low-valent d-block and f-block metal complexes to reduce CO₂ to oxalate. Horn, B., Limberg, C., Herwig, C. & Braun, B. Nickel(I)-mediated transformations of carbon dioxide in closed synthetic cycles: reductive cleavage and coupling of CO₂ generating Ni^(I)CO, Ni^(II)CO₃ and Ni^(II)C₂O₄Ni^(II)) entities. Chem. Commun. 49, 10923-10925, doi:10.1039/C3cc45407j (2013) reported the use of β-diketiminate-based nickel(I) complexes to reduce CO₂ to CO or C₂O₄ ²⁻ in two closed synthetic cycles. A significant limitation of the Horn et al. system was its use of KC₈, one of the strongest reducing agents available, to reduce Ni(II) to Ni(I).

Angamuthu, R., Byers, P., Lutz, M., Spek, A. L. & Bouwman, E. Electrocatalytic CO₂ Conversion to Oxalate by a Copper Complex. Science 327, 313-315, doi:10.1126/science.1177981 (2010) reported a binuclear copper(I) complex that can reduce CO₂ to oxalate, forming a tetranuclear copper(II) oxalate complex. Oxalate was then released by electrolysis, using lithium perchlorate as the supporting electrolyte, to complete the electrocatalytic cycle. The complex contained an amino-acid-derived ligand that bound two Cu atoms. Two of the complexes reacted with four CO₂ molecules to form two oxalates: 2Cu₂L+4CO₂→Cu₄L₂(C₂O₄)₂.

Crowley, J. D.; Bandeen, P. H., A multicomponent CuAAC “click” approach to a library of hybrid polydentate 2-pyridyl-1,2,3-triazole ligands: new building blocks for the generation of metallosupramolecular architectures. Dalton Trans. 2010, 39, 612-623; doi: 10.1039/B911276F discloses a CuAAC reaction for generating alkyl, benzyl or aryl linked polydentate pyridyl-1,2,3-triazole ligands from the corresponding halides, sodium azide, and alkynes. Complexes with Ag(I) were described.

Other methods to reduce CO₂ include electrochemical or photochemical processes. In the Bocarsly “liquid light” approach, CO₂ reacts with an electrochemically-reduced solution of a heterocyclic amine such as pyridine. In photochemical reduction, some or all of the energy needed for CO₂ reduction is supplied by light.

Reductive dimerization of carbon dioxide to oxalate (C₂O₄ ²⁻) converts an environmental pollutant into a more useful organic compound. There is an unfilled need for improved, economical methods to reduce CO₂ to oxalate. If a suitable chemistry could operate rapidly and cleanly enough, then it could be used to capture CO₂ from the atmosphere or from other chemical processes (e.g. combustion, cement manufacture). Oxalate and oxalic acid have many uses, including in extractive metallurgy, as mordants in dyeing processes, as bleaching agents, as miticides, and as reagents in various synthetic processes. In very large quantities, oxalate may also simply be used to sequester CO₂ in solid form, e.g. as CaC₂O₄.

DISCLOSURE OF THE INVENTION

We have discovered a novel composition and method for capturing CO₂. A pollutant/greenhouse gas can be converted into a valuable organic compound using a nontoxic reducing agent. CO₂ may be economically captured from the atmosphere, from products of combustion, or from byproducts of various chemical processes.

The novel method reduces CO₂ in a three-step reaction cycle, in which a binuclear metal-organic compound converts CO₂ to oxalate in a redox cycle under mild conditions. In one embodiment, a novel copper(II) complex [Cu₂L₂X₂]²⁺ (L=m-xylylene-bis(pyridyltriazole); X=NO₃, Cl) is reduced to its Cu(I) counterpart. This reduction may be performed either electrochemically or with a mild reducing agent such as sodium ascorbate. The [Cu₂L₂X₂]²⁺ complex selectively reacts with CO₂ to reduce it to oxalate, in an oxalate-bridged binuclear complex [Cu₂L₂(μ-C₂O₄)]²⁺. The bound oxalate ion may be released as oxalic acid by treatment with a strong acid such as HCl or HNO₃, thus regenerating the “empty” [Cu₂L₂X₂]²⁺ complexes and completing the cycle.

The novel metal-organic system will reduce CO₂ to oxalate under mild reaction conditions. The binuclear Cu complex provides a unique environment that promotes binding and reduction of two CO₂ molecules to produce one oxalate. Although CO₂ is fixed relatively slowly, the Cu(I) dimer selectively reacts with CO₂ instead of O₂, a distinct advantage for a simple-to-operate process. Oxalic acid is readily released from the product.

The binuclear metallacyclic copper complex selectively captures CO₂ from air (or other streams) and reduces it to oxalate in an oxalate-bridged complex. This complex releases oxalic acid when treated with dilute mineral acid, regenerating the “empty” macrocycle and completing the cyclic process. The complex reacts both with pure CO₂, and with CO₂ in concentrations typical of those in air.

In an alternative embodiment, we have used vitamin C (sodium ascorbate) as a reducing agent to yield the copper(I) complex that may then be used to reduce CO₂. Sodium ascorbate is a milder reagent and is easier to use than many other reducing agents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts syntheses of complex 1 and complex 2.

FIG. 2 depicts cyclic voltammograms for complex 1 and complex 2.

FIG. 3 depicts a portion of the UV-Vis absorption spectrum of a 3.76 mM solution of complex 1 with increasing amounts of added sodium ascorbate.

FIG. 4 depicts a portion of the UV-Vis absorption spectrum of a 0.20 mM solution of complex 1 with increasing amounts of added sodium ascorbate.

FIG. 5 depicts spectral changes over time during reduction of CO₂ to oxalate.

FIG. 6 depicts a portion of the UV-Vis absorption spectrum of a 0.12 mM solution of complex 3 over time during its reaction with CO₂.

FIGS. 7(a) and (b) depict the IR spectra of [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂ and [Cu₂(m-xpt)₂(μ-¹³C₂O₄)](PF₆)₂, respectively.

FIG. 8 depicts the difference between the IR spectra of [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂ and [Cu₂(m-xpt)₂(μ-¹³C₂O₄)](PF₆)₂.

METHODS Example 1. Equipment and Materials

All commercially available reagents and solvents were purchased from Aldrich or Alfa Aesar, and were used without further purification. ¹H Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker AV-400 MHz spectrometer. ESI mass spectra were measured on an Agilent 6210 instrument. FTIR spectra were recorded on a Bruker Tensor 27 spectrometer in ATR mode. M-H-W Laboratories (Phoenix, Ariz.) performed elemental analyses. UV-visible spectra were recorded on an Aviv 14DS spectrometer. The samples were prepared in sealed flasks, and spectra were recorded using cuvettes connected to the flasks.

Cyclic voltammetry measurements were performed using a Princeton Applied Research Model 273A potentiostat/galvanostat with Power Suite 2.53 software. The measurements were carried out in 0.1 M Bu₄NP F₆ in dimethylformamide (DMF), with a glassy carbon working electrode, Ag/AgCl reference electrode, and Pt wire counter electrode.

The ligand m-xpt was synthesized following the procedure of Pokharel, U. R., Fronczek, F. R. & Maverick, A. W. Cyclic pyridyltriazole-Cu(II) dimers as supramolecular hosts. Dalton Trans. 42, 14064-14067, doi:10.1039/C3dt52208c (2013).

Example 2. [Cu₂(m-xpt)₂(NO₃)₂](PF₆)₂, 1

To a stirred solution of Cu(NO₃)₂.3H₂O (0.612 g, 2.54 mmol) in acetonitrile (75 mL), m-xpt (1.00 g, 2.54 mmol) in chloroform (50 mL) was added dropwise. The reaction mixture was stirred at room temperature for 2 hours. The precipitate was collected by filtration, washed with acetonitrile and chloroform, and dried to yield a blue solid [Cu₂(m-xpt)₂](NO₃)₂](NO₃)₂ (1.57 g, 1.35 mmol). This solid was dissolved in water (200 mL), and an aqueous solution of NH₄PF₆ (1.32 g, 8.09 mmol) was added. The mixture was stirred for 10 min and filtered. The precipitate was collected, washed with water, and dried in air to give compound 1 (1.84 g, 84%) as a light-blue powder. ESI-MS: m/z 1349.078, [Cu₂(m-xpt)₂(PF₆)₃]⁺ (calcd 1349.082). Anal. Calcd for [Cu₂(m-xpt)₂(NO₃)](PF₆)₃H₂O: C, 36.93, H, 2.68, N, 16.64. Found: C, 36.88, H, 2.92, N, 15.40.

Complexing the ligand m-xpt with Cu(NO₃)₂ gave the dimeric macrocycle [Cu₂(m-xpt)₂(NO₃)₂](NO₃)₂. Although the distance between the two Cu centers in this compound was appropriate for small-molecule guests, the compound was insoluble in common organic solvents. To improve solubility and to widen the scope of host-guest chemistry, we replaced two of the nitrate anions with the more hydrophobic PF₆ ⁻. Metathesis gave [Cu₂(m-xpt)₂(NO₃)₂](PF₆)₂, Compound 1.

A stirred solution of complex 1 (200 mg, 0.14 mmol) in 20 mL of DMF was reduced to [Cu₂(m-xpt)₂](PF₆)₂, Compound 3, by adding sodium ascorbate (41 mg, 0.21 mmol) under nitrogen for ca. 1 hour.

The amount of nitrogen seen in the elemental analysis was consistently lower than expected. We postulate that this observation may be due to the presence of species such as [Cu₂(m-xpt)₂](PF₆)₄ in the product mixture. When the product was crystallized by vapor diffusion of diethyl ether into its solution in DMF with benzene, we isolated [Cu₂(m-xpt)₂(NO₃)₂](PF₆)₂, 1.

FIG. 1 depicts the synthesis of complex 1, [Cu₂(m-xpt)₂(NO₃)₂](PF₆)₂. Cu . . . Cu distance as measured by x-ray diffraction was 6.843(2) Å

Example 3. [Cu₂(m-xpt)₂Cl₂](PF₆)₂, 2

Complexing m-xpt with CuCl₂ gave the dimeric macrocycle [Cu₂(m-xpt)₂Cl₂]Cl₂. This compound was also insoluble in common organic solvents, and we replaced the chloride anions with PF₆ ⁻. Metathesis gave [Cu₂(m-xpt)₂Cl₂](PF₆)₂, Compound 2.

To a stirred solution of [Cu₂(m-xpt)₂Cl₂]Cl₂ (2.00 g, 1.86 mmol) in water (300 mL), excess NH₄PF₆ (1.82 g, 11.2 mmol) was added. The mixture was stirred for 10 minutes. The precipitate was collected by filtration, washed with water, and dried to give complex 2 (2.05 g, 86%) as a blue-green solid. A crystalline product was obtained from DMF by vapor diffusion of diethyl ether. ESI-MS: 1239.085 [Cu₂(m-xpt)₂(PF₆)₂Cl]⁺ (calcd 1239.087). Anal. Calcd for [Cu₂(m-xpt)₂Cl₂](PF₆)₂.3DMF: C, 42.55, H, 3.84, N, 17.79, Cl, 4.74. Found: C, 42.92, H, 3.49, N, 17.92, Cl, 5.21.

Example 4. [Cu₂(m-xpt)₂](PF₆)₂, 3

To a stirred solution of complex 1 (200 mg, 0.14 mmol) or complex 2 (178 mg, 0.14 mmol) in DMF (20 mL), sodium ascorbate (41 mg, 0.21 mmol) was added under N₂. After 1 hour, the yellow solution was slowly diffused with diethyl ether under nitrogen for 2 days. The resulting solid precipitate was collected by filtration, washed with diethyl ether, and dried to give complex 3 (164 mg, 85% from complex 1; 156 mg, 81% from complex 2). ¹H NMR (DMSO-d₆, 400 MHz): 5.79 (s, 8H, CH₂), 7.43-7.49 (m, 12H, Ar), 8.12 (br, 8H, Ar), 8.43 (br, 4H, Ar), 9.23 (br, 4H, triazole). ¹³C NMR (DMSO-d₆, 125 MHz): 54.3, 122.1, 125.1, 125.9, 128.8, 129.3, 130.1, 136.0, 139.3, 146.0, 147.1, 149.5. ESI-MS: 1059.171 [Cu₂(m-xpt)₂(PF₆)]⁺ (calcd 1059.154). Anal. Calcd for [Cu₂(m-xpt)₂](PF₆)₂: C, 43.82, H, 3.01, N, 18.58. Found: C, 43.58, H, 3.16, N, 17.92.

As we had observed for compound 1, compound 3 also gave microanalyses that were low in nitrogen. The addition of a small amount of ether to the formula gave better agreement; however, there were no signals attributable to ether or to other impurities in the compound's NMR spectrum.

Example 5. [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂, 4

We tested the selectivity of the reaction of complex 3 with CO₂, by generating the Cu(I) dimer. The yellow solutions that resulted from the previous syntheses were exposed to air to react with atmospheric CO₂, and also to allow slow evaporation of the DMF solvent. After 4 to 5 days, we isolated complex 4 in high yield (96% from complex 1; 69% from complex 2). We observed green octahedral crystals of [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂, 4, synthesized starting from [Cu₂(m-xpt)₂(NO₃)₂](PF₆)₂, 1 (crystal size ca. 0.3 mm). Both green and blue-green crystals were formed when complex 2 was used as the starting material. This conversion demonstrated selective reaction of [Cu₂(m-xpt)₂]²⁺, 3, with CO₂ over O₂.

Both types of crystals were characterized by X-ray crystallography. The green crystals were found to be the desired oxalate-bridged dimer 4. The blue-green crystalline product was the starting Cu(II) dimer, [Cu₂(m-xpt)₂Cl₂](PF₆)₂. Both were washed with acetonitrile, which dissolved the blue-green crystals. Dimer 4 was collected by filtration (194 mg, 96% from complex 1; 138 mg, 69% from complex 2). FTIR (cm⁻¹): 1670 (s), 1645 (s), 1610 (s), 1454 (s), 839 (vs), 785 (vs), 715 (vs). Anal. Calcd for [Cu₂(m-xpt)₂(μ-C₂O₄)(PF₆)₂].2DMF: C, 43.37, H, 3.50, N, 17.51. Found: C 43.48, H 3.52, N 17.58. The Cu . . . Cu separation, 5.4213(7) Å and the distances within the bridging oxalate ligand (C—C 1.544(7) Å; C—O 1.248(3) Å), were similar to those that have been observed for other C₂O₄ ²⁻-bridged copper complexes.

Example 6. [Cu₂(m-xpt)₂(μ-¹³C₂O₄)](PF₆)₂

The Cu(I) complex 3 derived from complex 1 was transferred into a sealed flask connected to a cuvette. The reaction mixture was purged with nitrogen using four alternating cycles of vacuum and nitrogen. The reaction mixture showed no changes in the UV-vis spectrum after 48 hours. The N₂ was then replaced with ¹³CO₂. The yellow solution progressively turned green over a period of 128 hours, as ¹³CO₂ was added periodically to maintain the pressure at ca. 1 atm. After the reaction was complete, the solution was poured into a watch glass and left to crystallize by slow evaporation of DMF. The crystalline solid was washed with acetone.

Example 7. [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂, 4a

As a complementary reaction to that displacing bound oxalate from complex 4, we also tested the ability of empty macrocycles to accept oxalate ion as a guest. The reaction of complex 1 and 2 with tetrabutylammonium oxalate in acetonitrile yielded [Cu₂(m-xpt)₂(μ-C₂O₄)]²⁺, as confirmed by single crystal X-ray analysis.

We prepared [Cu₂(m-xpt)₂(μ-C₂O₄)]²⁺ from Cu(II) and oxalate as follows: To a stirred solution of complex 1 (200 mg, 0.14 mmol) or complex 2 (178 mg, 0.14 mmol) in acetonitrile (20 mL), tetrabutylammonium oxalate (80 mg, 0.15 mmol) was added. The mixture was stirred for 30 minutes and then left to crystallize by slow evaporation of the solvent. Yellow-green crystals formed, and were then washed with acetonitrile and air-dried to give 4a (178 mg, 87% from complex 1; 147 mg, 72% from complex 2). X-ray analysis showed that these crystals were the acetonitrile solvate of [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂, 4a (Cu . . . Cu=5.462(2) Å), which is isostructural with the DMF solvate 4. (See Table 2 for data and refinement parameters.)

Example 8. [Cu₂(m-xpt)₂(H₂O)₂](NO₃)₄, 5

Bound oxalate was removed from [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂, 4 to regenerate the “empty” Cu(II) macrocycle [Cu₂(m-xpt)₂]⁴⁺. Crystallization from DMF/H₂O gave the “empty” host complexes [Cu₂(m-xpt)₂Cl₂]Cl₂ and [Cu₂(m-xpt)₂(H₂O)₂](NO₃)₄, 5, from the reaction of complex 4 with HCl and HNO₃, respectively. The yields of empty complexes after acid treatment were nearly quantitative: 94% (HCl) and 96% (HNO₃).

HCl(aq) (8 eq) or HNO₃(aq) (8 eq) (2 mL of 0.32 M, 0.64 mmol) was added to a stirred suspension of complex 4 (100 mg, 0.07 mmol) in methanol (50 mL). The solid dissolved, giving a greenish-yellow solution (HCl) or a blue solution (HNO₃), which deposited a green or blue-green precipitate after stirring for an additional 3 hours. The mixture was filtered and the solid was re-dissolved in a mixture of water and DMF (4:1 v/v). The solution was poured into a watch glass and left to stand for slow evaporation. After 4 to 5 days, a blue-green (HCl) or blue (HNO₃) crystalline product had formed. These crystals were washed with ca. 5 mL methanol and were air-dried. X-ray analysis of the products revealed the formation of empty Cu(II) macrocycles: [Cu₂(m-xpt)₂Cl₂]Cl₂.4DMF (with HCl; 92 mg, 94%) or [Cu₂(m-xpt)₂(H₂O)₂](NO₃)₄.4DMF, 5 (with HNO₃; 81 mg, 96%).

The filtrate from the above reaction was evaporated to dryness. The solid was dissolved in water (2 mL). The solution was neutralized with KOH(aq) and extracted with chloroform. The aqueous phase was acidified with HCl and again evaporated to dryness to give oxalic acid as a white solid which was characterized by ¹³C NMR (163.2 ppm) and FTIR spectroscopy (ν_(CO)=1668 cm⁻¹; with ¹³CO₂, 1642 cm⁻¹).

[Cu₂(m-xpt)₂(H₂O)₂](NO₃)₄, 5, formed by removal of oxalate from [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂, 4 had a Cu . . . Cu distance=7.2441(8) Angstrom Anal. Calcd for [Cu₂(m-xpt)₂(H₂O)₂](NO₃)₄.2H₂O: C 42.76, H 3.59, N 22.66. Found: C 42.38, H 3.55, N 21.86.

Results Example 9. Cyclic Voltammetry

Cyclic voltammetry of complex 1 and complex 2 in DMF showed quasi-reversible waves at ca. 0.28 V vs. Ag/AgCl (−0.27 V for complex 1 and −0.28 V for complex 2 vs. Fc/Fc⁺). FIG. 2 shows cyclic voltammograms of [Cu₂(m-xpt)₂(NO₃)₂](PF₆)₂ and [Cu₂(m-xpt)₂Cl₂](PF₆)₂ in DMF containing 0.1 M Bu₄NPF₆. Data were recorded on a static glassy carbon disc working electrode with a Pt wire auxiliary electrode and Ag/AgCl reference electrode at 25° C. at a scan rate of 50 mV s⁻¹.

We investigated the reactivity of the Cu(I) dimers obtained by reducing complex 1 or 2. For chemical reduction of Cu(II) to Cu(I), we used sodium ascorbate to produce Cu(I) catalysts in situ for an azide-alkyne cyclization reaction. Treatment of complex 1 or 2 with sodium ascorbate in DMF under N₂ gave a yellow copper(I) complex. During the reduction of complex 1 or 2 to Cu(I), the Cu(II) d-d electronic absorption band disappeared, and an intense new band at 384 nm appeared (FIGS. 3 and 4). We attributed the 384 nm (ε=1.05×10⁴ M⁻¹ cm⁻¹) band to a metal-to-ligand charge transfer transition in the macrocyclic Cu(I) complex [Cu₂(m-xpt)₂](PF₆)₂, 3. This new compound was surprisingly stable in air for several days in the solid state.

Solutions of 3, generated by in situ reduction of complex 1 or 2 with sodium ascorbate in DMF, reacted with CO₂(g) to produce the oxalate-bridged Cu(II) dimer [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂, 4. The progress of the reaction was monitored by electronic absorption spectroscopy. We examined both the appearance of the Cu(II) d-d band in dimer 4 (λ_(max)=751 nm, ε=98 M⁻¹ cm⁻¹) and the disappearance of the Cu(I) metal-to-ligand charge transfer band at 384 nm, under CO₂ maintained at 1 atm. The reaction was nearly complete after 128 h.

FIG. 3 depicts a portion of the UV-Vis spectrum of a 3.76 mM solution of [Cu₂(m-xpt)₂(NO₃)₂(PF₆)₂, 1 in DMF with increasing amounts of added sodium ascorbate: 0 to 1.25 eq.

FIG. 4 depicts a portion of the UV-Vis spectrum of a 0.20 mM solution of [Cu₂(m-xpt)₂(NO₃)₂(PF₆)₂, 1 in DMF with increasing amounts of added sodium ascorbate: 0 to 1.25 eq. λ_(max)=384 nm.

FIG. 5 depicts spectral changes over time during reduction of CO₂ to oxalate. A 3.76 mM solution of [Cu₂(m-xpt)₂](PF₆)₂, 3, generated in situ from complex 1 in DMF using sodium ascorbate, was kept under N₂ for 48 h, followed by oxidation by CO₂. The spectra, measured after 8 h and then every 24 h to 128 h, demonstrated gradual formation of [Cu₂(m-xpt)₂(μ-C₂O₄)]²⁺, 4 (λ_(max)=751 nm).

FIG. 6 depicts a portion of the UV-Vis spectrum over time of a 0.12 mM solution of [Cu₂(m-xpt)₂](PF₆)₂, 3, in DMF during its reaction with CO₂: 0 h (at top) to 112 h at bottom).

Example 10. IR Spectra

IR spectra of [Cu₂(m-xpt)₂(μ-C₂O₄)](PF₆)₂ and [Cu₂(m-xpt)₂(μ-¹³C₂O₄)](PF₆)₂ are shown in FIGS. 7(a) and (b), respectively. The IR spectrum of dimer 4 showed ν_(CO)=1670 cm⁻¹; an analogous measurement with ¹³CO₂ gave ν_(CO)=1651 cm⁻¹. A difference spectrum showing ν_(13CO)=1651 cm⁻¹ is depicted in FIG. 8.

Example 11. X-Ray Data Collection and Structure Determination

Intensity data were collected at low temperature on a Bruker Kappa Apex-II DUO CCD diffractometer fitted with an Oxford Cryostream chiller. MoKα (λ=0.71073 Å) radiation with a TRIUMPH curved graphite monochromator was used for 2, 4, and 4a. CuKα (λ=1.54184 Å) radiation from an I μS microfocus source with QUAZAR multilayer optics was used for 1 and 5. Data reduction included absorption corrections by a multiscan method, with SADABS, or TWINABS for 5.

Structures were determined both by direct methods and by difference Fourier techniques, and were then refined by full-matrix least squares using SHELXL-97. All non-hydrogen atoms were refined anisotropically, except for the minor component in a disordered nitrate in 1 and 5, C atoms in 2, and the minor component of the disordered PF₆ ⁻ anion in 1 (vide infra).

The crystal structure of dimer 4 contained dimeric cationic complexes with crystallographically imposed 2/m (C_(2h)) symmetry. For 2, low data quality did not allow anisotropic refinement of the C atoms. Disordered solvents were removed using the SQUEEZE procedure for 2, resulting in non-integral solvent stoichiometry. Normal refinement procedures for 2 led to several unreasonable distances within the m-xpt ligands. Therefore, for the final refinement, the two sets of ligand atoms were restrained to yield similar bond distances and angles.

For 4 and 4a, the PF₆ ⁻ anion was disordered in two orientations. A single solvent molecule (DMF for the crystals prepared by reaction of Cu(I) (3) with CO₂; CH₃CN for the crystals prepared by reaction of Cu(II) (1 or 2) with oxalate) was disordered across a mirror plane.

The crystal of 5 was a nonmerohedral twin by 180° rotation about real axis [−1 0 1]. Refinement was versus HKLF5 data, and the twin components were present approximately 52/48%. This structure had a disorder involving a nitrate ion in two positions and a partially occupied water molecule associated with one of them. All H atoms were placed in idealized positions, except for water hydrogen atoms. Water H atoms were refined with restrained O—H distances, but the H atoms of the water molecule in the nitrate/water disorder of 5 could not be located.

Example 12. Crystal Data and Structure Refinement Parameters

TABLE 1 Crystal Data and Structure Refinement Parameters for 1 and 2. Compound 1 2 deposition no. CCDC 1000457 CCDC 1000458 formula [Cu₂(m-xpt)₂(NO₃)₂](PF₆)₂ · [Cu₂(m-xpt)₂Cl₂](PF₆)₂ · 3.5CH₃CN 4.44C₃H₇NO M 1329.93 1601.17 crystal system Monoclinic Orthorhombic space group P2₁/c Pna2₁ a/Å 11.4090(16) 13.810(2) b/Å 12.8455(18) 19.380(3) c/Å 23.411(3) 24.711(3) β/deg 108.575(5)  90 V/Å³ 3252.3(8)  6613.6(16) Z 2 4 T/K  100.0(5)  90.0(5) D_(calc)/g cm⁻³ 1.358 1.608 crystal 0.35 × 0.17 × 0.10 0.04 × 0.14 × 0.31 dimensions/mm Radiation CuKα MoKα θ limits/deg 3.98-59.20 1.65-25.71 reflns, measd/ 22792/4639/3982 52940/12074/7203 unique/obsd F(000) 1340 3278 μ/mm⁻¹ 2.066 0.871 R_(int) 0.0468 0.0525 R[I>2σ(I)] 0.1013 0.0830 R_(W) (all data) 0.3540 0.2384 GOF 1.624 1.053

TABLE 2 Crystal Data and Structure Refinement Parameters for 4, 4a, and 5. Compound 4 4a 5 deposition no. CCDC 984468 CCDC 984469 CCDC 984470 formula [C₄₆H₃₆Cu₂N₁₆O₄](PF₆)₂ · [C₄₆H₃₆Cu₂N₁₆O₄](PF₆)₂ · [C₄₄H₄₀Cu₂N₁₆O₂](NO₃)₄ · 2C₃H₇NO 4CH₃CN 2C₃H₇NO · 3.34H₂O M 1440.12 1458.14 1406.50 crystal Orthorhombic Orthorhombic Monoclinic system space group Cmca Cmca P2₁/n a/Å 24.1471(17) 24.461(7) 11.4361(13) b/Å 11.7107(7)  11.959(3) 22.691(3) c/Å 20.5949(12) 20.008(5) 11.9848(13) β/deg 90 90 106.847(5)  V/Å³ 5823.8(6)   5853(3) 2976.6(6) Z 4 4 2 T/K  100.0(5)  100.0(5)  100.0(5) D_(calc)/g cm⁻³ 1.642 1.655 1.569 Crystal 0.45 × 0.17 × 0.04 0.30 × 0.22 × 0.13 0.12 × 0.19 × 0.21 dimensions mm radiation MoKα MoKα CuKα θ limits/deg 1.69-30.05 2.04-30.23 4.32-69.67 reflns, measd/ 44218/4357/3592 31270/4418/3607 52513/14803/12358 unique/obsd F(000) 2928 2960 1455 μ/mm⁻¹ 0.891 0.886 1.691 R_(int) 0.0310 0.0447 0.0741 R[I>2σ(I)] 0.0527 0.0346 0.0637 R_(W) (all data) 0.1531 0.0900 0.1928 GOF 1.033 1.031 1.024

In one embodiment, a compound in accordance with the present invention is a binuclear compound having the structure [Cu₂L₂(μ-C₂O₄)_(p)]X_(n) ^(m+) wherein:

-   -   L is m-xylylenebis(pyridyltriazole);     -   X is a monovalent anion; wherein if n is 2 or 4, the various X         anions may be the same or different; as illustrative examples, X         may be chloride, other halide, halogen, nitrate, or any other         common monovalent anion;     -   p is 0 or 1;     -   n is 0, 2, or 4; and     -   m is 0, 2, or 4.

In another embodiment, a compound in accordance with the present invention is a binuclear compound having the structure [Cu₂L₂(μ-C₂O₄)_(p)]X_(n) ^(m+) wherein:

-   -   X is a monovalent anion; wherein if n is 2 or 4, the various X         anions may be the same or different; as illustrative examples, X         may be chloride, other, halogen, nitrate, or any other common         monovalent anion;     -   p is 0 or 1;     -   n is 0, 2, or 4;     -   m is 0, 2, or 4;     -   L is:

R¹ may be used to modify the properties of the internal cavity where CO₂ binds, or to control access to the cavity. R¹ may be, for example, H, C₁-C₄ substituted or unsubstituted alkyl, —NH₂, —OH, —CO₂H; R², R³, and R⁴, particularly R³, may be used to enhance the solubility of the complexes in certain solvents. For example, R³=OCH₂CH₂CH₂CH₃ is expected to increase solubility in nonpolar solvents such as toluene. R², R³, and R⁴ may, for example, be independently chosen from H, substituted or unsubstituted C₁-C₈ alkyl, substituted or unsubstituted aryl, —OH, —OC_(k)H_(2k+1), —O(CH₂CH₂O)_(k)CH₃, —O(CH₂CH₂O)_(k)C₂H₅, —N(CH₃)₃ ⁺, —SO₃ ⁻, —NO₂, —CN (k=1-8); R⁵, R⁶, R⁷, and R⁸ may be used to modify solubility, or to modify the electron-donating ability of the ligand as it coordinates to the Cu ions, or both. Modifying the electron-donating ability may be useful in altering the reducing strength of the Cu(I) complexes, which will in turn affect the rate of reaction of Cu(I) with CO₂. R⁵, R⁶, R⁷, and R⁸ may, for example, be independently chosen from: H; —OCH₃, —N(CH₃)₂ (electron-releasing); halogen, —NO₂, —CN (electron-withdrawing); or substituted or unsubstituted C₁-C₈ alkyl, substituted or unsubstituted aryl, —OH, —OC_(j)H_(2j+1), —O(CH₂CH₂O)_(j)CH₃, —O(CH₂CH₂O)_(j)C₂H₅, —N(CH₃)₃ ⁺, —SO₃ ⁻, (j=1-8).

The complete disclosures of all references cited in the specification are hereby incorporated by reference in their entirety, as is the complete disclosure of priority application Ser. No. 61/890,403. Also incorporated by reference is the complete disclosure of the following work by the present inventors, including its associated supplemental information: Pokharel, U. R., Fronczek, F. R. & Maverick, A. W. Cyclic pyridyltriazole-Cu(II) dimers as supramolecular hosts. Dalton Trans. 42, 14064-14067, doi:10.1039/C3dt52208c (2013). In the event of an otherwise irresolvable conflict, however, the disclosure of the present specification shall control. 

What is claimed:
 1. A binuclear compound having the structure [Cu₂L₂(μ-C₂O₄)_(p)]X_(n) ^(m+) wherein: p is 0 or 1; n is 0, 2, or 4; m is 0, 2, or 4; X is a monovalent anion; wherein if n is 2 or 4, the various X anions may be the same or different; and L is:

wherein: R¹ is chosen from the group consisting of H, C₁-C₄ substituted or unsubstituted alkyl, —NH₂, —OH, and —CO₂H; R², R³, and R⁴ are independently chosen from the group consisting of H, substituted or unsubstituted C₁-C₈ alkyl, substituted or unsubstituted arene or heteroarene radical, —OH, —OC_(k)H_(2k+1), —O(CH₂CH₂O)_(k)CH₃, —O(CH₂CH₂O)_(k)C₂H₅, N(CH₃)₃ ⁺, —SO₃ ⁻, —NO₂, and —CN; wherein k is 1, 2, 3, 4, 5, 6, 7, or 8; R⁵, R⁶, R⁷, and R⁸ are independently chosen from the group consisting of H, —OCH₃, —N(CH₃)₂, halogen, —NO₂, —CN, substituted or unsubstituted C₁-C₈ alkyl, substituted or unsubstituted arene or heteroarene radical, —OH, OC_(j)H_(2j+1), —O(CH₂CH₂O)_(j)CH₃, —O(CH₂CH₂O)_(j)C₂H₅, —N(CH₃)₃ ⁺, —SO₃ ⁻, —NO₂, and —CN, wherein j is 1, 2, 3, 4, 5, 6, 7, or 8; and wherein: said compound has the property that it can cycle between a Cu(I) form of the compound and a Cu(II) form of the compound, as a function of redox potential; and wherein: the Cu(I) form of the compound has the property that it can react with carbon dioxide to reduce carbon dioxide to oxalate, whereby the compound is oxidized to the Cu(II) form of the compound.
 2. The compound of claim 1, wherein L is m-xylylenebis(pyridyltriazole).
 3. The compound of claim 1, wherein said compound is selected from the group consisting of [Cu₂L₂(μ-C₂O₄)]²⁺; [Cu₂L₂(μ-C₂O₄)](PF₆)₂; [Cu₂L₂(NO₃)₂](PF₆)₂; [Cu₂L₂(NO₃)₂](NO₃)₂; [Cu₂L₂](PF₆)₂; [Cu₂L₂Cl₂]Cl₂; [Cu₂L₂Cl₂](PF₆)₂; [Cu₂L₂]²⁺; and [Cu₂L₂]⁴⁺.
 4. The compound of claim 3, wherein L is m-xylylenebis(pyridyltriazole).
 5. A method for reducing carbon dioxide to oxalate; said method comprising the sequential steps of: (a) reacting carbon dioxide with the Cu(I) form of the compound of claim 1 to produce oxalate complexed to the Cu(II) form of the compound of claim 1; (b) reacting the oxalate complex from step (a) with a mineral acid to liberate oxalate from the Cu(II) form of the compound of claim 1; (c) reducing the Cu(II) form of the compound of claim 1 from step (b) to regenerate the Cu(I) form of the compound of claim
 1. 6. A method for reducing carbon dioxide to oxalate; said method comprising the sequential steps of: (a) reacting carbon dioxide with the Cu(I) form of the compound of claim 2 to produce oxalate complexed to the Cu(II) form of the compound of claim 2; (b) reacting the oxalate complex from step (a) with a mineral acid to liberate oxalate from the Cu(II) form of the compound of claim 2; (c) reducing the Cu(II) form of the compound of claim 2 from step (b) to regenerate the Cu(I) form of the compound of claim
 2. 7. A method for reducing carbon dioxide to oxalate; said method comprising the sequential steps of: (a) reacting carbon dioxide with the Cu(I) form of the compound of claim 3 to produce oxalate complexed to the Cu(II) form of the compound of claim 3; (b) reacting the oxalate complex from step (a) with a mineral acid to liberate oxalate from the Cu(II) form of the compound of claim 3; (c) reducing the Cu(II) form of the compound of claim 3 from step (b) to regenerate the Cu(I) form of the compound of claim
 3. 8. A method for reducing carbon dioxide to oxalate; said method comprising the sequential steps of: (a) reacting carbon dioxide with the Cu(I) form of the compound of claim 4 to produce oxalate complexed to the Cu(II) form of the compound of claim 4; (b) reacting the oxalate complex from step (a) with a mineral acid to liberate oxalate from the Cu(II) form of the compound of claim 4; (c) reducing the Cu(II) form of the compound of claim 4 from step (b) to regenerate the Cu(I) form of the compound of claim
 4. 